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Paper presented at 2012 IEEE-PES General Meeting, 22 July, San Diego, USA 2012. This material is posted here with the permission of the IEEE. Such permission of the IEEE does not in any way imply IEEE endorsement of any of ABB’s products or services. Internal or personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional purposes or for creating new collective works for resale or redistribution must be obtained from the IEEE by writing to [email protected].
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Abstract – This paper describes voltage source converter
(VSC) offshore technologies for expansion of point-to-point transmission systems to HVDC offshore grids. Today several gigawatt wind farm projects are underway or have been proposed in the North Sea, Baltic Sea and Atlantic Ocean.
These first offshore VSC stations has been installed on oil and gas platforms for power from shore in 2005 or using structures that are lifted on site for connection of wind farms in 2010. New and innovative modular solutions for converter offshore platforms in the gigawatt-range have been developed.
As the offshore wind generation increases it makes sense to connect the offshore converters into a multi-terminal grid for better utilization of the transmission systems and connection to balancing power. The key technological cornerstones to plan and start building the first parts of HVDC offshore grids are available today and the gaps for deployment of a full-fledged offshore grid are outlined.
Index Terms — AC-DC power converters, HVDC transmission, Offshore installations, Wind farms
I. NOMENCLATURE ESS – Energy Storage System MTDC – Multi-terminal high voltage direct current LCC – Line Commutated Converter UHVDC – Ultra-high HVDC VSC – Voltage Source Converter
II. INTRODUCTION HE first commercial High Voltage Direct Current
(HVDC) was installed to power the Island of Gotland from shore by a 96 km 100 kV subsea cable in 1954. Since this first 20 MW transmission system more than 100 HVDC systems have been installed world wide. Some of the major technical breakthroughs have been the introduction of Thyristor-based valves in the 70’s, the first Multiterminal schemes in the early 90’s, IGBT-based converters in the latter 90’s and the UHVDC transmission systems operation at 800 kV with transmissions capacities above 6 GW.
HVDC using the voltage source converter technology
E.M. Callavik is with ABB Power Systems, HVDC in Västerås, Sweden, (e-mail: [email protected])
M.P. Bahrman is with ABB Grid Systems North America in Raleigh, NC, USA (e-mail: [email protected]).
P. Sandeberg is with ABB Power Systems, Offshore Wind Connections, in Västerås, Sweden, (e-mail: [email protected])
(VSC) is the technology to connect remote offshore wind cluster to shore [1]. Since 2005 HVDC voltage source converters has been successfully deployed to power offshore oil and gas platforms from shore and since 2010 also to transmit power from offshore wind generators to shore. Already about 5 GW of transmission of offshore wind located in the North Sea has been awarded to be connected to the German shore up to 2015. As offshore wind generation increases it makes sense to connect the offshore converters into a MTDC grid for better utilization of the transmission systems and enable connection to balancing power.
The key technological cornerstones needed to plan and start building the first parts of such HVDC grids are available today including new platform technology to facilitate the location of large HVDC stations offshore. These offshore grids can be progressively introduced using HVDC point-to-point connections based on the latest flexible voltage source converters. Examples of required HVDC components and system solutions needed to realize a continental-wide grid connecting remote power sources such as large-scale wind farms with central European load centers are load flow control, protection methods and DC breakers. Several ongoing CIGRE working groups are working on common guidelines on such technical solutions, e.g. B4-52, B4-57, B4-58, B4/B5-59 and B4-60 as well as the Cenelec TC8X Study group on Technical Guidelines for HVDC Grids.
In addition to closing the technical gaps, also regulatory aspects and harmonization of grid codes are needed on national and international levels. Several CIGRE working groups are working on these issues (B4-52 and B4-56).
III. HVDC-BASED POWER-FROM-SHORE HVDC based on the VSC technology has been used in
offshore applications since 2005 when the first two compressors at the Troll A oil and gas platform in the North Sea was powered from hydro resources by a 70 km subsea link from the Norwegian shore [2]. The successful installation has been followed by a similar design for the Valhall platform in 2011. The Valhall transmission system includes onshore and offshore converter stations, which convert ac power from the 300 kV sub-station at Lista on the Norwegian shore to dc power at 150 kV, transmit it through the 292 km sub-sea dc cable and convert it back to ac at 11 kV at the offshore platform to feed the entire Valhall field [3]. Recently Statoil
Technology Developments and Plans to Solve Operational Challenges Facilitating the HVDC
Offshore Grid Magnus Callavik, Michael Bahrman, and Peter Sandeberg
T
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awarded two additional VSC links to the Troll A platform to be in operation 2015. The main reasons for powering the platforms from shore are to reduce costs, improve operation efficiency of the field and to minimize emission of green house gases. HVDC is the only rational transmission solution at these sub sea distances and power level. Since IGBT-based VSC-HVDC (HVDC Light™) is self-commutated, the converters require no existing AC voltage to operate. VSC-HVDC is now an established solution for power-from-shore applications where it operates as electrical drives together with the cable wound, high-voltage motor on the platform.
IV. HVDC-BASED OFFSHORE POWER-TO-SHORE Borwin 1 in the North Sea outside Germany is the first
VSC-HVDC for power-to-shore from offshore wind farm [4]. It was put into operation 2010 to enable transmission of 400 MW. The wind farm cluster will consist of 80 wind generators of 5 MW scheduled for full operation in 2012. The offshore converter is located about 130 km from the coast. The generators feed power into a 36 kV AC cable system which is transformed to 154 kV for the HVDC Light® offshore station. The receiving station is located at Diele, 75 km from the coast, where the power is injected into the German 380 kV grid. Hence a total of 205 km of sea and underground cables have been deployed successfully, partly in sensitive natural protected areas.
Dolwin 1 and 2 are further examples of North Sea wind projects to be commissioned during 2013 and 2015, respectively. These VSC systems will be operating at a DC voltage of 320 kV, transmitting 800 and 900 MW of peak power, respectively. The Dolwin transmission systems use the cascaded two level converters (CTL) technology that has reduced losses to about 1 % per converter and reduced the need for AC filters [5]. A simplified single line diagram of the Dolwin 1 is outlined in Fig. 1. As references overview presentations of HVDC links provided by ABB are readily available on internet [6].
V. OFFSHORE AND PAN-CONTINENTAL HVDC GRIDS A high penetration of intermittent and remote wind energy
creates a demand for a stronger power grid for transmission to load centres and efficient power balancing. Real-time generation and demand balancing is traditionally achieved by deploying various types of ancillary services such as frequency reserves through flexible adjustments of generation outputs and responsive loads. A HVDC Grid enables rapid power flow control including change of flow direction in the time range of seconds over long distance with relatively low power losses. Furthermore, HVDC enables a distributed connection to load centres located far from the optimum source location for power generation. Interconnections between major cities and industrial sites with non-simultaneous peak and low loads spreading over large distances are ways of creating a secure and power-balanced system. Unused transmission capacity, available when wind farms are running on less than peak generation can be used for balancing and energy trading purposes between AC grids.
Fig. 1. Dolwin 1 simplified single line diagram. Offshore station depicted in the top with gas insulated switchgear (GIS) on the converter platform and GIS at the collector platform. Onshore station depicted in the bottom with DC choppers to handle DC faults.
VI. PRIMARY HVDC GRIDS BUILDING BLOCKS
A. Regional HVDC Grids Regional multi-terminal HVDC Grid has one HVDC zone
meaning that it is not possible to separate the faulty part of the DC network at DC earth faults. Instead all circuit breakers on the AC side of the converters are opened; thereafter DC switches are used to isolate the faulty part if needed. Finally the system is re-energized. These types of networks can be built today. The limitations of such a grid is not in geographic size, as a matter of fact it could cover very long distance, e.g. several hundred or thousands of kilometers and connect to several non-synchronized AC networks, such as point-to-point HVDC transmission system of today. The limitation of a regional grid is on the maximum in-feed power that can be lost in the connected AC network at the connection point, or the sum of several terminals connected in the same AC network. However, it should be noted that other faults such as tripping of a converter are handled by separate isolation from the DC grid of that particular converter by DC switches, while the rest of the HVDC network stay in operation.
Classic or line-commutated converter (LCC) HVDC multi-terminal schemes have been available for more than two decades, exemplified by the 2000 MW New England-Hydro Quebec MTDC commissioned in 1992. Recently an 800 kV four converter MTDC was announced to be in operation in 2014-2015 in India to connect North East India with Agra rated at a maximum power of 8000 MW, supplying hydro-power based electricity to 90 million people [6].
LCC multi-terminal networks have some limitations in flexibility of power reversal compared to voltage source converter (VSC) grids. VSC DC links can change power by changing current direction, whereas LCC networks must do a voltage polarity switch by using DC switchyard components. LCC HVDC also needs to connect to AC grids with a certain
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level of short circuit capacity and always needs reactive power compensation.
These restrictions with the LCC technology make it unsuitable for larger HVDC grids. The recent developments in VSC technology, such as increased power capacity, compactness and loss reduction of cascaded or multilevel VSC-based HVDC technology [5], points towards that VSC will be the technology used to build off-shore HVDC Grids and large HVDC networks [1]. As shown in Paragraphs III and IV, several VSC systems are also operating offshore.
Indeed the first European regional HVDC VSC-based networks are also being discussed and planned for offshore and onshore applications in Europe. Recent point-to-point connections have been required to be grid-enabled to enable future expansion by connections to other DC systems. Also traditional HVDC links planned in regions where the density of HVDC is increased tend to ask for grid-enabled or multi-terminal ready solutions, clearly showing the intention of a future solution with multi-terminal HVDC networks.
B. Inter-regional HVDC Grids The required components for planning an inter-regional
HVDC Grid are in a development stage that makes planning a possible reality today. Examples of the key building blocks are discussed below in Section C.
In an area with several point-to-point HVDC links, i.e. with two converter stations, there are several benefits to connect these converters to each other in a network. One important aspect is the possibility of increased power balancing which is discussed in more detail in the later part of this paper. In addition the regional MTDC has two straightforward benefits with respect to investment and operational expenditures. To begin with the number of converters, which is one of the most costly components in an HVDC link, is reduced by one in a three node grid compared to building two point-to-point connections, i.e. a reduction with 25%. To replace five equal point-to-point connections in a radial manner only requires a six-node radial grid, i.e. the number of converters can be reduced from ten to six, cf. Fig. 2. . Secondly, even if the losses in VSC-HVDC has been reduced significantly during recent years down to ca. 1 %, a reduction of the number of AC-DC conversion steps will give a gain of approximately 1 % in a three-node grid and 4 % in a six node radial grid.
It has also been shown that HVDC grids give a better utilization of large central energy storage systems (ESS) compared to several distributed smaller ESS [7] with the same overall installed capacity.
C. Building blocks for Inter-regional HVDC Grids An inter-regional grid implies that the size of the grid is
large and the operational requirements specify that only parts of the HVDC network should be disconnected during a DC earth fault. If this shall be possible during load currents an HVDC circuit breaker is needed.
The inter-regional grids may emerge from a situation when regional grids close to each other should be linked together. If these regional grids have been built with varying HVDC voltage, DC-DC converters are needed to level the voltage.
When long HVDC links are being built to cross several
regions from a remote generation site to a load center, it may be justified to feed-out or tap a portion of the capacity at an intermediate location. This may be at a load center that needs only a part of the overall capacity. It may also be a local generation site that could be connected to feed-in power to the long link. Such tapping or feed-in/feed-out converters are generally considered to be less than 20% of the overall capacity. These converters can be built using the present topology, but it may also be possible to optimize the design.
Finally the HVDC grid could be built using either overhead lines, plastic-insulated cables, mass-impregnated cables or combinations of these three solutions. Examples of all technologies have been deployed in projects with VSC-HVDC for point-to-point connections. At present the limit for plastic cables is at 320 kVDC, while mass-impregnated cables can be used up to 500 kVDC. For overhead lines the LCC-HVDC has reached 800 kV. Hence it is likely that converter and switchyard solutions for such voltage levels can be reached using the VSC-HVDC technology.
Fig. 2. Benefits with HVDC Grids compared to point-to-point connections. The conventional point-to-point HVDC (left) compared to a five node radial multi-terminal HVDC grid (right). As the number of HVDC links increases it becomes logical to connect them into one overlay grid. The number of converters is reduced from eight to five, reducing initial capital expenditures. In addition operational converter losses are reduced by roughly 1 % per converter, i.e. 3 % in this example.
1) HVDC Circuit breakers
As discussed in the review by Frank [8] “the acceptance of HVDC networks with respect to efficiency, reliability, and controllability will strongly depend on the availability of HVDC circuit breakers”. However it should be noted that HVDC circuit breakers are not needed for regional grids where the impact of shutting down the grid during DC earth faults have limited effect on the connected AC system. But for sure the existence of DC breakers will be a prerequisite for building and planning larger grids. Therefore ABB has piloted a power electronic-based HVDC circuit breaker with capacity to interrupt DC load currents in the millisecond time range. The present design has negligible conduction losses while preserving fast current interrupting capability. [9].
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2) DC – DC Converters DC-DC converters for high voltage applications will not be
needed in the near future, since the main application will be to interconnect several existing HVDC Grids if there are built with varying DC voltage. It is not until a number networks with varying voltage must be in operation. There are several suggestions on how to design such converter topologies. One of the key challenge lies on optimization of the solution from a cost point of view and other market requirements. Although the final design is not available today the technology will be available within the required timeframe.
VII. OFFSHORE HVDC CONVERTER PLATFORMS With trends going towards huge offshore power plants
located remotely on sites far out in a harsh and unforgiving environment, the suppliers can also expect an increased demand for platforms which has the capacity not only to give shelter to large AC-substations or HVDC converter station but also to act as hubs for operation and maintenance personnel. To meet these demands a highly innovative, robust and scalable platform has been designed addressing issues such as e.g. efficient production and easy installation requiring only a minimum of offshore works without the need of a heavy lift vessel or jack-up operations. It is a design which is flexible in respect to installation programs as it is possible to put in place all around the year in the North Sea waters and elsewhere.
A. Gravity based structure platform main design and layout The platform concept is a self-installing steel gravity base
structure (GBS) intended to be floated to site and installed by ballasting, depicted in Fig. 3.
The construction resembles a traditional twin pontoon structure, consisting of two submersed pontoons, six columns supporting the deck structure and a bracing system connecting and stabilizing the platform parts. The modular approach is shown in Fig. 4.
The topside has two principal decks called main deck (lowest deck) and intermediate deck. These decks are part of the platform structural design and are split into different rooms. In addition there is also a so called weather deck utilized as listed below.
The main deck (lowest deck) mainly contains ABB supplied equipment such as e.g. HV GIS, shunt reactors, main HV transformers with dedicated arrestors, converter reactors and other HVAC and HVDC components.
The intermediate deck contains converter IGBT-valves in six rooms and a free fall lifeboat.
The weather deck will typically contain MV and LV transformers and switch-boards, batteries and distributions,
HVDC and platform control, safety and telecom central equipment, converter IGBT-valve cooling system, emergency generator, workshops, platform control room and emergency room. Furthermore there will be two cranes and HVA/C systems for weather deck rooms and inert gas housing.
The modular design approach also allows for a permanent accommodation module to be installed onto the weather deck.
The size of the module is customized and defined by client specification and local and global regulations, standard and norms.
The top of the columns (box top) are utilized for platform secondary and utility systems equipment as e.g. seawater system (filters, coolers, sub-merged pumps), cooling medium coolers and pumps, main HVA/C systems, diesel system (tanks, pumps etc), drainage systems, firewater pumps, fresh water tank and pumps, etc.
Power and fibre cables are guided from sea bottom to cable splicing area in two of the column tops via dedicated J-tubes.
Fig. 3. A 3-D model of HVDC hub for the Dolwin 2 project. The gravity structure is to be in place offshore 2014
Fig. 4. Modularization of a gravity based structure platform. The topside structure is the same for all concepts, whereas the substructure is sized for water depth and power rating
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B. Transportation and installation of platform Prior to load out, all equipment on the platform will be
installed and commissioned as far as reasonable at the yard. The transportation to the installation site is done by tug
boats. The design fulfils all relevant standards with respect to seaworthiness meaning that the installation procedure has a low weather dependency. In practice this means that the platform can stay out at sea even under rough weather conditions and wait for the weather to stabilize enough to allow for the actual ballasting operation.
Upon arrival to the site, the towing lines will be re-arranged to suitable length and angles. One large or two smaller additional tugs will be mobilized locally and connected to the platform, e.g. Fig. 5.
Provided that the weather criteria for installation are fulfilled, the positioning will commence. The platform will be gradually ballasted down until the pontoons are just above the seabed. When an acceptable position is confirmed, the platform position will be secured by further ballasting down to achieve skirt penetration. The installation operation will be monitored by ROVs.
For permanent ballasting solid ballast (e.g. gravels) will be filled into the lower part of all six columns. The type and amount of ballast and the method to be used for filling the columns will be determined during detail engineering.
The installation method described above requires only a minimum of hook up works offshore and no heavy lift vessel nor are noisy piling operations required.
The offshore commissioning is limited to energization and trial run-after installation of the HV-cable.
Fig. 5. Transportation, installation and positioning of gravity based structure
VIII. CONNECT REMOTE WIND WITH ONSHORE LOADS
A. Offshore HVDC grid studies in Europe Several parallel projects are underway in Europe to study
the potentials and challenges with offshore grids. In a recent paper [10], the “Offshore grid” study compared DC grid hubs with versus individual connections, tee-in connections, and hub-to-hub connectors. To connect the future planned 129 GW of wind in the North and Baltic Seas an investment of €92B is estimated if only point-to-point connections are used. With a HVDC Grid approach 15% of investment could be saved. It was concluded that the financial analysis clearly pointed towards the benefit of an offshore grid where transmission capacities are split between wind farms.
Other ongoing studies all conclude that a new overlay grid will be needed in the long run to connect remote renewables with load centers and balancing power. Examples such as the ten year network development plan (TYNDP) for Europe, the first report of from the Friends of the Supergrid (FOSG), German Grid Masterplan, Desertec Industrial Initiative (Dii) and Medgrid reports will shortly be updated and summarized at the presentation.
B. Offshore HVDC applications in America There are abundant, high-quality, developable wind
resources across the American Great Plains. The highest penetration of wind generation and development focus has been concentrated in this large, sparsely-populated region. Significant transmission investment is required to deliver energy from these resources to the closest major load centers, e.g. approximately 400 km distant in Texas, Iowa and Minnesota. Much longer transmission distances are required to deliver energy from prime wind resources in the western Dakotas, Montana and Wyoming.
Over 50% of the US population lives in the coastal zones where the population density, especially in the mid-Atlantic and Northeast regions, is highest. Transmission distances from the Great Plains to coastal load centers can approach 2000 km and must pass through congested areas. These factors make development of offshore wind resources, especially in the mid-Atlantic and Northeast US., attractive for future consideration.
Consequently several offshore wind projects have been proposed for this region. The Atlantic Wind Connection [11] consists of an offshore backbone HVDC transmission system, with capacity to connect up to 7000 MW of wind capacity on the mid-Atlantic continental shelf, with multiple delivery points on the mainland grid.
Much of the development of transmission technology for integration of offshore wind can also be applied for accessing remote isolated wind resources located on land. The State of Hawaii for example plans to significantly increase its penetration of renewable resources on Oahu. Options include bringing in hundreds of MW of wind generation from the neighboring islands of Molokai and Lanai which have only about 10 MW of local peak load. Even some of the major land-based wind developments are located remotely where
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existing transmission infrastructure is deficient thereby making HVDC transmission with VSC an attractive and economic alternative.
C. Regulation and harmonization of HVDC grid code By no surprise the first VSC-HVDC grids already in
planning, such as the Swedish South-West link [11-12], are developed by single transmission system (TSO) operators. Here the TSO embed the HVDC system in the existing AC grid. One of the key challenge to solve for larger HVDC Grids are development of uniform grid codes and harmonization of renewable support schemes to allow generation of renewable energy in one country and transmitting it by HVDC to another. Efforts are underway in several working groups and various cross-European initiatives.
IX. CONCLUSIONS Several VSC-HVDC links are now in operation in offshore
application in the harsh North Sea. Following the rapid increase in power transmission capacity and loss-reduction, several new projects up to the gigawatt range is in the construction phase. Such HVDC links can be expanded to smaller regional multiterminal DC networks, which already is in planning in Europe and North America. The benefits of larger intercontinental grids have been shown in several ongoing studies. Hence the first Multiterminal HVDC networks can be seen as initial building blocks on the way for a larger offshore grid.
X. REFERENCES [1] E. Koldby, M. Hyttinen, Challenges on the Road to an Offshore HVDC
Grid, Nordic Wind Power Conference 2009, Bornholm Denmark 2009 [2] P. Jones, L. Stendius, The Challenges of Offshore Power System
Construction Troll A, Electrical Power Delivered Successfully to an Oil and Gas Platform in the North Sea, EWEC, 2006 (available at www.abb.com)
[3] B. Westman, S Gilje, .M Hyttinen, Valhall Re-development Project, Power from Shore, PCIP Europe, 2010, (available at www.abb.com)
[4] J. Kreusel, The future is now, ABB Review, pp 40-44, 4/2008 [5] B. Jacobson, P. Karlsson, G. Asplund, L. Harnefors, T. Jonsson, VSC-
HVDC Transmission with Cascaded Two-Level Converters, B4-110, CIGRE 2010
[6] http://www.abb.com/hvdc, search e.g. North East – Agra or Dolwin 1 [7] M. Callavik, C. Yuen, J. Åhström, HVDC Supergrids for Continental
Wide Power Balancing, 10th International Workshop on Large Scale Integration of Wind Power, October 2011
[8] C. M. Franck, HVDC Circuit Breakers: A Review Identifying Future Research Needs, IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 26, NO. 2, APRIL 2011
[9] J. Häfner, B. Jacobson, Proactive Hybrid HVDC Breakers - A key innovation for reliable HVDC grids, B4 0264, CIGRE Bologna 2011
[10] P. Kreutzkamp, J. De Decker, N. Picot, OffshoreGrid: Techno-Economic Model for Future Offshore Electricity Transmission, 10th International Workshop on Large Scale Integration of Wind Power, October 2011
[11] http://atlanticwindconnection.com [12] http://www.svk.se/sydvastlanken [13] http://tdworld.com/projects_in_progress/business_in_tech/svenska-
kraftnat-hvdc-tech/
XI. BIOGRAPHIES
Dr. Magnus Callavik (M’2009) was born in Sweden on April 19, 1969. He graduated from the Royal Technology of Technology, Stockholm (MSc 1994, PhD 1998), and was a Research Fellow at Stanford Research Institute in California during 1994. He holds an Executive MBA from Stockholm School of Economics (2009) and is a certified project management professional (PMP) He was employed at ABB in 1999. During the last years he has held R&D and laboratory test management positions at ABB Corporate Research. Since 2010
he is program manager for HVDC Grids at ABB Power Systems. He is a board member of KIC Innoenergy and the Swedish National Committee for IEC and Cenelec.
Michael Bahrman has been responsible for system analysis, system design, project engineering and project management for various ABB HVDC and FACTS projects in North America. This experience includes multi-terminal HVDC control development, testing and commissioning of the Quebec-New England HVDC system. He is currently responsible for business development of HVDC systems in North America. He has been
active in assessing transmission alternatives for large scale renewable energy projects. Prior to joining ABB, Mr. Bahrman was with Minnesota Power where he held positions as Transmission Planning Engineer, HVDC Control Engineer and Manager of System Operations. Mr. Bahrman holds a degree in Electrical Engineering from Michigan Tech, is a registered professional engineer in the state of Minnesota and is a member of IEEE.
Mr. Sandeberg has been working in the power system industry for more than 20 years and within the renewable sector since 2005. He is currently holding a position as Technology Manager at ABB’s unit for offshore wind connections. He has a MSc degree in electrical engineering from the University of Linkoping and an Executive MBA from Stockholm School of Economics. After his studies he started at ABB working with HVDC and FACTS and since 2005 he has been involved in Offshore Wind Connections.
